In the computer industry, cable assemblies are used for many purposes including power distribution, low speed signal communication, and high speed signal communication. In a high performance computer system, specialized cable assemblies are used to transmit the high speed signals from processor to processor, processor to memory, and processor to I/O devices. These specialized high speed cable assemblies are mechanically and electrically connected to the printed circuit boards that support each of these key functions.
The time of flight or propagation delay for the high speed signals between these functions is critical to the overall performance of the computer system. The total propagation delay for these high speed signals is determined by the length of the circuit traces and wires used to transmit the signals from one point to another and the dielectric properties of the materials surrounding these circuit traces and wires. As computer systems increase in performance, the propagation delay for the high speed signals can be decreased by reducing the total path length for the signals to travel as well as improving the dielectric properties for the printed circuit boards and cable assemblies. An improvement in the propagation delay can be achieved by decreasing the board space required to connect to the high speed cable assemblies which in turn reduces the overall size of the board and the internal wiring along with the total space required for the system. The net result of the size reduction (or increase in packaging density) is a reduction in the signal propagation delay and an opportunity to increase the system performance.
The packaging density of a cable-to-board connector system is measured by the total number of signal and ground contacts divided by the total area used to attach and connect the cable assembly to the printed circuit board. This area is determined by the size of the cable mounting hardware and the distance or pitch between the rows and columns of contacts in the connector system. Decreasing the pitch between contacts is a common approach used to increase the packaging density of a cable connector system. As the pitch of the contacts is decreased, the size of the contacts is also decreased and smaller contacts are more prone to physical damage and quality defects.
Smaller spring contacts are more susceptible to stress relaxation of the spring material that can cause reliability problems in a connector system. Besides the material and quality limitations of using smaller contacts, there are many other limiting factors for increasing the packaging density of a cable connector including the physical size of the discrete high speed wires along with the wireability requirements of the printed circuit board. Another technique that is used for increased packaging density requires a connector system design that allows the individual cable assemblies to be stacked side by side and mounted perpendicular to the printed board surface.
Almost all cable assemblies use a separate connector mechanism to join the cable wires to a printed circuit board. The few exceptions use a flexible polyimide or mylar circuit instead of wire cables and the integral contacts are part of the wiring on the flex circuit. The separate connector mechanism can be mounted to the printed circuit board using a variety of fabrication and assembly techniques including using pins soldered into plated through holes, pins with compliant sections pressed into plated through holes, and surface mounted leads attached to pads on the surface of the printed circuit board. These techniques may require separate processing steps to attach the connectors to the printed circuit board. Separate connector components and processing steps will increase the cost of the total assembly.
These fabrication and assembly techniques are additional limiting factors for the packaging density of a cable connector system. Typical plated thru holes are drilled into the printed circuit board on a fixed rectangular 0.100.times.0.100 inch grid or an interstitial or staggered 0.050.times.0.050 inch grid. These packaging density limitations are determined by the diameter of the hole along with the width and spacing of the internal board wiring. Although surface mounted cable connectors eliminate the need for the plated thru holes in the printed circuit board, the packaging density is limited by the surface mounted lead configuration and the area required for the solder pads on the surface of the printed circuit board.
In order to provide a reliable electrical connection between the cable assembly and the printed circuit board, it is necessary to provide a sliding or wiping action between the contact surfaces in the cable connector. This wiping action allows the contacts to push aside debris and penetrate any oxides or films between the two contact surfaces. Typical two piece cable connectors use a pin (male) and spring loaded socket (female) contact geometry. As the pin is inserted into the socket, the mating contact surfaces wipe against each other. Another design consideration for a reliable electrical connection between a cable assembly and the printed circuit board is the number of independent contact interfaces for each signal or power connection. Two or more independent contact interfaces provide redundancy.
In addition, we would note that there are publications and patents of which we are aware which are listed below with a brief discussion of each of the publications and patents.